CO2, NOx, black exhaust, harmful particulate material, etc. contained in an exhaust gas discharged from an engine generator, which is used for example in a civil engineering work, an electric work, a building work, or the like may invite an environmental pollution. In addition, the noise from the engine may also cause disturbance in sleep in the middle of night, thus exerting an influence on a life environment. Accordingly, there has been expected a practical realization of a power supply apparatus utilizing a secondary battery without an exhaust gas and noise (a battery-powered generator). A lithium ion secondary battery may be taken as an example of a future technology having a high degree of expectation.
FIG. 9 shows a general circuit diagram of a power supply apparatus utilizing such a secondary battery. According to the power supply apparatus of FIG. 9, pushing a start switch causes a control unit to close a main contact, and an inverter is activated, thus permitting to discharge an electric current to a load. Alternatively, pushing a stop switch causes the control unit to open the main contact to quit the power supply apparatus. During a non-operational period of time of the power supply apparatus, the electric current as charged in the secondary battery is consumed as a standby electricity. The apparatus is connected to a charge unit during a charging step constitutes a closed circuit between the apparatus and the charge unit to perform a charge. When the charging step is carried out so as to exceed a predetermined amount of charge, the control unit quits the charge from the charge unit based on information on the secondary battery, as acquired from the secondary battery.
The secondary battery (for example, a lithium battery, a nickel hydride battery, etc.) has a remarkably large amount of energy, and any abnormal situation may lead to increase in an internal pressure, thus inviting a problem of blowing out a high-temperature gas. Therefore, there has been a strong demand for a technique to exclude such a risk. Particularly, the lithium battery has a low tolerance for overcharge and over-discharge, and has a low overcharge tolerance of about 150% to about 200%, while a lead battery generally has an overcharge tolerance of about 500%. In addition, an occurrence of overcharge may cause the melting of an electrode material due to an internal short-circuit of the battery, thus being at risk for the blowing out of the contents. Further, the over-discharge in which the voltage is equal to or less than a predetermined minimum voltage may causes the battery itself to produce heat, leading to a hazardous situation in the same manner.
The main contact for opening or closing the direct current electricity of the secondary battery as shown in FIG. 9 may cause a contact fusion due to use for a long period of time, with the result that the discharge continues although the control unit makes a control of opening the main contact, leading to the over-discharge. In general, an alternating current circuit has a periodical 0-V point (a zero-cross) of a voltage waveform, and accordingly an arc-discharge may terminate shortly and the contact fusion may not occur. However, in a case of the direct current electricity such as the secondary battery, a constant voltage may not cause the arc-discharge to terminate shortly, and use for a long period of time or use with a high current may cause the arc-discharge easily. The over-discharge of the secondary battery due to the contact fusion may deteriorate performance of the battery and cause a reverse reaction of the battery, thus inviting the above-mentioned serious situation.
For example, Japanese Patent Provisional Publication No. H08-182115 (hereinafter “Patent Document 1”) discloses an art to detect a contact fusion of a contact and prevent a battery of an automobile from running out. The art disclosed in Patent Document 1 relates to a power supply control apparatus of an electric automobile, in which a fusion detection unit compares, when a main conductor is shut down by a control unit, a detection voltage of a voltage detector that detects a voltage across the terminals of an auxiliary battery with a comparison voltage, and judges, when the voltage exceeding the above-mentioned comparison voltage has been detected, that a fusion occurs in the main conductor, thus causing a DC-DC convertor to continue surely an operation thereof, when the fusion occurs in the main conductor. It is therefore possible to make a sure judgment of occurrence of a fusion based on continuation or discontinuation of the operation of the convertor, thus enhancing a reliability of a detection of the fusion. In addition, the DC-DC converter is caused to quit when the fusion detection unit detects the occurrence of fusion of the main conductor, thus preventing the main battery from running out even when an operator has failed to take any action to avoid the fusion.
The apparatus utilizing a secondary battery is generally provided with a circuit (a battery management system) that monitors and controls a secondary battery from the outside, or a circuit (an ECU: Electronic Control Unit) that monitors internally the apparatus itself, in order to use safely the secondary battery. In an equipment such as an electric automobile on which the secondary battery is mounted, an electricity for maintaining such a system itself is supplied from the secondary battery itself or the auxiliary battery, and the electricity for the circuit is always consumed when the apparatus is not even operated.
In an electric automobile in which a battery having a large capacity of from about 10 kWh to abut 30 kWh is mounted and an auxiliary battery such as a lead battery is mounted, it is considered that there is no possibility of over-discharge when the apparatus is not operated. However, for example in a portable power supply apparatus or an industrial equipment having a capacity of from 1 kWh to less than 10 kWh, in which a secondary battery is merely mounted, no use of it for a long period of time would lead to consumption of electricity for the system from the secondary battery, thus may cause a problem of over-discharge of the secondary battery. It is conceivable that an auxiliary battery is provided in the same manner as an electric automobile, in order to solve the above-mentioned problem. However, the auxiliary battery such as a lead battery has a short service life and a heavy weight, and is not suitable for the portable power supply apparatus or the like.